Converge is similar to an old mechanic called Sunburst. However, that mechanic only cared about adding counters. With Converge it’s possible we’ll see some cards that actually will benefit from you putting all 5 colors into them. There seems to be a great deal of design space for this mechanic. With the Eldrazi already being seen in Red, Blue, and Black (Grixis colors), this mechanic (so far) would seem to be tailored towards them. But it certainly is playable in other three-color (or more) combos, as well.
Radiant Flames is certainly Standard-playable. It does require three-color builds to be highly effective, which limits its scope. But cards that spread out a lot of damage for a minimal mana investment like this are going to be played. As we've seen with cards in the past such as Pyroclasm and Anger of the Gods, this is definitely a useful card.